The problem of drift of the capacitive sites toward a substrate voltage level is not known to have been addressed in the prior art with circuitry to neutralize the problem. The problem is reduced when the sense time is short. Certain fabrication techniques eliminate the application of a voltage potential to the substrate, and that normally eliminates the drift. Where the problem eists and the drift not reversed, the drift is toward a status which turns on access-switch transistors to the memory cells. In time each cell originally storing ground potential has its access transistor biased near threshold. This seriously degrades the sensing operation during normal read and write, as the ground level memory cells after drift will pass charge from a sense line. In addition to draining charge, the access transistor also passes noise at every site at which drift to near threshold has occurred.
This invention reverses the drift by a circuit system which applies a threshold potential to the gate of the access tranistors while their drains are driven high to thereby take out the negative charge. The employment of a potential controlled to threshold is employed in a memory circuit in U.S. Pat. No. 4,204,277 to Kinoshita. This functions, however, to hold bit lines to a given level to reduce oscillations.
The drift toward substrate potential and its negative consequences is eliminated by a circuit design which is practical and need not require substantial power. Because of the low power requirements, the subject circuit is well suited to use a standby battery for automatic protection of information in memory during loss of power.